Body-coupled communications (BCC) or body-based communication has been proposed as a promising alternative to radio frequency (RF) communication as a basis for body area networks (BANs). BCC allows exchange of information between a plurality of devices which are at or in close proximity of a body of a human or an animal. This can be achieved by capacitive or galvanic coupling of low-energy electric fields onto the body surface. Signals are conveyed over the body instead of through the air. As such, the communication is confined to an area close to the body in contrast to RF communications, where a much larger area is covered. Therefore, communication is possible between devices situated on, connected to, or placed close to the body. Moreover, since lower frequencies can be applied then typically applied in RF-based low range communications, it opens the door to low-cost and low-power implementations of BANs or personal area networks (PANs). Hence, the human body is exploited as a communication channel, so that communication can take place with much lower power consumption than in standard radio systems commonly used for BANs (e.g. ZigBee or Bluetooth systems). Since BCC is usually applied in close proximity to the body, it can be used to realize new and intuitive body-device interfaces based on contact or proximity. This creates possibilities for many applications in the field of identification and security.
BCC can be technically realized by electric fields that are generated by a small body-worn tag, e.g., being integrated to a credit card or another suitable device attached to or warn in close proximity to the body. This tag capacitively or galvanically couples a low-power signal to the body. Sometimes this body-coupled communication is referred to as “near-field intra-body communication”. BCC is a wireless technology that allows electronic devices on and near the human body to exchange digital information through capacitive or galvanic coupling via the human body itself. Information is transmitted by modulating electric fields and either capacitively or galvanically coupling tiny currents onto the body. The body conducts the tiny signal to body mounted the receivers. The environment (the air and/or earth ground) provides a return path for the transmitted signal.
FIG. 1 shows an exemplary body communication system structure, where data signals are transmitted via couplers placed near or on the body. These couplers transfer the data signal, either galvanically or capacitively, to the body. In the example of FIG. 1, one coupler or electrode provides ground potential GND and the other coupler or electrode is used for transmitting/receiving a signal S. More specifically, transmission from a transmitter (TX) 10 to a receiver (RX) 20 over a human arm is depicted. Generally, every node can in principle act both as transmitter and receiver, i.e., as a transceiver (TRX), and communication can take place from everywhere on the body.
BAN technology is standardized by the 802.15.6 Task Group of IEEE. This standardization task group has an ambition to include on-body and in-body communications in the standard. The details described below in connection with the embodiments can be applied to on-body communications and in-body communications as well. In the following discussion, it will be referred to the case of in-body communications between medical implants and external programmers. Of course, the invention can as well be applied to any kind of data communication.
A communication band from 402 to 405 MHz with a channel spacing of 300 kHz has been set aside for wireless communications involving medical implants. This band is called the Medical Implant Communication Service (MICS) band. It is envisaged that the IEEE 802.15.6 Task Group will adopt packet-based communications in the standard.
MICS-band communications can be applied to various applications, such as electronic pills (e-Pill), implantable drug delivery, deep brain simulation, capsule endoscope, etc. Such applications cover a wide range of data rates, e.g., from about 100 bps (integrated drug delivery) to about 1.5 Mbps (capsule endoscope). A capsule endoscope is a medical implant that is swallowed by a patient and takes images of the gastrointestinal system when it reaches the desired location inside the body. Such endoscope can be used to help doctors in medical diagnosis. The images taken by the endoscope are transmitted from inside the body to the outside through a wireless radio link. To support endoscope applications, 64 QAM (Quadrature Amplitude Modulation) at 250 k symbols per second may for example be used with a square-root raised cosine (SRRC) pulse shape of rolloff factor 0.15 in MICS-band communications. The signal is thereby contained within the 300 kHz band. The use of such high transmission rate and high modulation format requires accurate frequency synchronization, timing synchronization and channel estimation.
To achieve the desired accuracy, a data packet is required to be properly designed. The data packet consists of a preamble and a data payload. The preamble is a known sequence enabling a receiver to achieve frequency synchronization, etc. As an example, it may be estimated that a preamble length of 336 symbols is required. A lower number of symbols may result in unsatisfactory accuracy, so that it is not likely to reduce the preamble length. It is however noted that the preamble is a transmission overhead, so that a longer preamble reduces transmission efficiency, which is not desirable.
The payload or payload portion which follows the preamble in the data packet carries the data. It may have a payload length of 1024 symbols. Simulations have been performed on a receiver to decode packets with a payload length of 1024 symbols and a preamble length of 336 symbols. Received packets are routed through several receiver functions, such as coarse frequency offset estimation, timing synchronization, fine frequency offset estimation, channel estimation, channel equalization, frequency offset tracking, and finally symbol demodulation. At a signal-to-noise ratio (SNR) of 24 dB, the uncoded block error rate (BER), i.e., the BER without error correction coding, was found to be 0.0016, which is consistent with text book figures.
FIG. 3 shows a typical constellation diagram of the decoded symbols observed in some simulation runs. It depicts detected variations within the 64 possible states of the 64 QAM signal. This diagram serves as a reference constellation diagram that yields satisfactory decoding results, due to the fact that the 64 different constellations can be clearly discriminated or distinguished at the receiver.
Although the BER performance is satisfactory, the transmission efficiency of the data packets is only 1024/(336+1024) which corresponds to about 75%. The net data transmission rate is only 250 k symbols per second multiplied by 6 bits per symbol multiplied by 75%, which equals to 1.125 Mbps, quite far from the best data rate of 1.5 Mbps. It is thus desirable to increase transmission efficiency. This could be achieved for example, by increasing the payload length to 4096 symbols, which results in an efficiency of 4096/(336+4096) and corresponds to about 92%, which gives a net data transmission rate of 1.38 Mbps. However, an important problem is receiver performance, as explained in the following.
FIGS. 4a to 4c show samples of decoded-symbol constellation diagrams obtained in simulation runs with a payload length of 4096 symbols and a conventional receiver processing. The constellation diagram shown in FIG. 4a is symmetric and yields a satisfactory BER. On the other hand, the ones shown in FIGS. 4b and 4c are rotated and distorted, and thus lead to high BERs, since the constellations cannot be clearly discriminated or distinguished at the receiver. It has been found from a simulation of 1000 packets that the average BER is 0.0406, significantly greater than the BER for the case of a payload length of 1024 symbols.